The presence of oxide (scale) on the surface of steel strip, sheet, or breakdowns, is objectionable when such materials are to be further processed. For example, the oxide must be removed and a clean surface provided if satisfactory results are to be obtained from the hot-rolled sheet or strip in any operation involving deformation of the material. If the sheet is for drawing applications, removal of the oxide is essential, and its presence on the steel surface tends to shorten die life, cause irregular drawing conditions and destroy surface smoothness of the finished product. Oxide removal is also necessary to permit proper alloying or adherence of metallic coatings and satisfactory adherence when a non-metallic coating or paint is used.
In the production of cold-reduced steel sheet and strip, it is necessary that the oxide resulting during hot rolling the steel slab to breakdown form be completely removed before cold reduction to prevent lack of uniformity and eliminate surface irregularities.
The term "oxide" as used here refers generally to the chemical compounds of iron and oxygen formed on the surface of the steel by exposure to air while the metal is at an elevated temperature. "Scale" is specifically the oxidized surface of steel produced during hot working of steel. Hence, the oxide produced on steel surfaces in hot-rolling processes is known as mill scale. Chemical compounds thus formed are iron oxides FeO, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4.
Pickling is the process of chemically removing oxides and scale from the surface of a metal by the action of water solutions of inorganic acids. Considerable variation in type of pickling solution, operation and equipment is found in the industry. Among the types of pickling equipment may be mentioned the batch picklers, modified batch, non-or semi-continuous and continuous picklers.
The reaction occurring when steel or iron materials are immersed in dilute inorganic acid solutions includes the solution of metal as a salt of the acid and the evolution of hydrogen. Steel pickled in dilute hydrochloric acid and sulfuric-acid solutions is an example of this reaction, with the end products of reaction being, respectively, ferrous chloride and hydrogen and ferrous sulfate and hydrogen. Adherent films of oxides are undermined by the acid attack upon the scale on the base metal.
The rate of pickling is affected by numerous variables, including the steel-based constituents and type and adherence of oxide to be removed. Solution temperature and concentration, ferrous chloride or ferrous sulfate concentration, agitation, time of immersion and presence of inhibitors influence the rate of acid attack. Because of factors including pickling speed and efficiency, as well as reduced attack on the base metal, hydrochloric acid has effectively displaced sulfuric acid as the acid of choice in industrial pickling operations. While the rate of pickling increases in direct proportion to the concentration of the acid, the influence of temperature is much more pronounced. For example, in 15 percent sulfuric acid an increase in temperature over the range 70.degree. F. to 210.degree. F. doubles the pickling rate for each rise of 15.degree. or 20.degree. F. in temperature. Rate of solution of iron at 180.degree. F. is about five times the rate at room temperature. Certain metals, such as copper, chromium and nickel, retard the rate of pickling when they occur in the steel base, since the scale bearing these alloying metals inhibits acid attack. Silicon and aluminum form refractory-type oxides, which in turn lower the solubility rate of the oxide in the acid.
With the advent of continuous cold-reduction mills, it was necessary to design and develop suitable equipment to remove the oxides resulting from the oxides resulting from the continuous hot-rolling operation and prepare the hot-rolled breakdowns for cold reduction in coil form. This operation is typically performed in either a continuous or semi-continuous (push-pull) pickling line. The primary function of a continuous and semi-continuous pickling line, as of other pickling processes, is the removal of oxide from the steel surface. This serves to promote maximum reduction with a minimum of power to assure good roll life in the cold-reduction mills and to secure the increased surface density possible with cold work. The primary differences between a continuous pickle line and a non-or semi-continuous pickle line is that in a continuous line the tail of one coil is welded to the head of the next coil so that the strip is always in tension. In addition, continuous pickle lines generally require looping pits for providing strip storage space when brief delays arise at the strip charging end and for permitting a uniform rate of travel through the pickling tanks. An advantage of semi-continuous picklers is that they readily accommodate coil-to-coil changes in strip width and gauge with minimal down time.
The thickness of the oxide varies considerably on steel rolled on the hot-strip mill. Loose coiling permits greater atmospheric penetration into the wraps, with corresponding heavier oxide formation on the edge areas. Flexing of the steel in passing through the pickling line uncoiler and temper mill breaks this scale or oxide film and permits more rapid attack by the acid bath.
The non-continuous and continuous pickler has other advantages or supplementary functions. The product of the hot-strip mill is subject to fluting (formation of creases when the steel is bent or otherwise deformed) due to lack of springiness. Non-continuous and continuous pickling lines usually are equipped with suitable apparatus for cold working the material so that severe local strains are eliminated and fluting largely is prevented. The conventional pickling line permits inspection of the upper surface of the steel for defects and suitability for the next operation, as well as oiling of the steel as a protection against rusting and as an aid to cold reduction.
At the coil entry end of a typical semi-continuous or a continuous pickling line are facilities for handling and charging coiled product into the line. These usually consist of conveyors on which the coils are placed in proper sequence by overhead cranes, upenders in cases where the coil is delivered with the axis vertical, and a motor-driven integrated buggy and hoist for placing the coil in the uncoiling or pay-off equipment. The primary cold-working equipment called a "processor," or "flattener," integral with the uncoiling equipment, consists of a mandrel on which the coil is placed, a hold-down roll, and a series of smaller diameter rolls. After the coil is charged on the mandrel and the lead-end entered into the small diameter rolls, the hold-down roll is brought down and pressure applied to the material. This action alternatively flexes the steel around the rolls, thus effectively "breaking" the surface scale into numerous fine cracks, and increasing the available sub-oxide area for pickle attack. This flexing also cold works the steel enough to eliminate, in large part, the fluting tendencies of the hot-rolled steel. The group of small driven rolls immediately following the hold-down or breaker roll applies tension to the steel and also serves to straighten and flatten it. A stationary shear is located after the processor for the cropping and squaring of the coil ends.
In some pickling lines, an auxiliary or secondary scalebreaker is provided to break the scale even further than was achieved in the processor at the entry end, and thus increase the speed at which the line can be operated and still produce satisfactorily pickled strip. The secondary scalebreaker may be a machine similar to the entry-end processor, or it may be a two-high temper mill preceded and followed by a tension bridle at the entry and exit sides of the mill. Use of a two-high temper mill, for example, may result in extension of the strip on the order of 3 percent and increased is hardness 3 to 5 points on the Rockwell B scale.
The pickling zone usually consists of several individual acid-proof tanks located in series, comprising an effective immersion length of about 250 to 300 feet. While many lines have from three to five tanks, each about 70 to 80 feet long, some lines have only one long tank, divided by weirs into four or five sections. The strip is completely submerged under several inches of liquid acid bath as it travels through the tank or series of tanks forming the pickling zone. Automatic acid controls are available which monitor the HCl content in one or more tank sections of the line and automatically add acid to maintain a preset concentration. In a typical line the acid is added in the third and fourth tank section. The pickling solution then cascades over the top of the strip and counter to strip travel through shallow channels cut in the weirs between tank sections. The tanks are about four feet in depth and weir heights are decreased about an inch per weir from the exit entry end of the tanks. In a typical four tank line, weir heights would be on the order of 40, 39 and 38 inches in height from the exit to the entry end of the pickling tanks.
In hot-rolling operations, the steel strip is usually water cooled after hot-rolling by exposing the top surface of the strip to a stream or spray of water. The cooling water tends to pool on the top surface of the strip, thereby cooling the top surface at a faster rate than the bottom surface. And, since the rate of oxidation of steel is a proportional function of both time and temperature, scale formation is virtually always thicker on the bottom surface of the hot-rolled strip.
Consequently, in deep bath pickling tanks heretofore known in the art wherein the pickle liquor is introduced above rather than beneath the strip, the acid attack on the relatively thicker scale at the strip bottom surface is essentially passive in nature, hence resulting in slower than desirable line speeds in order to remove the scale from the bottom surface.
Following the acid tanks in a conventional pickle line are rinsing tanks consisting of cold-water spray rinse and, occasionally, a hot-water tank. The cold water rinses the acid carry-over from the steel. The hot-water rinse is a tank with an effective product immersion length of 15 to 20 feet. This tank completes the rinsing and by warming the steel, promotes flash drying prior to entering the succeeding set of pinch rolls. Situated between the final rinse tank and the pinch rolls are one, two or three banks of hot-air dryers operating at low pressures. Pinch rolls at the exit end of the pickling tanks control the speed of product travel and, in conjunction with the pinch rolls which provide back tension at the entry end of the line, help to maintain the proper loops in the tanks.
The delivery or exit end of the pickling line commonly has, in the order listed, a looping pit, pinch rolls, shear, oiler, recoiler and suitable supplementary equipment for conveying the finished product from the line. The pinch rolls preceding the shear are located so that product delivery to the shear is facilitated. Stitches, if present, are removed at this point, as well as short sections which inspection has shown to be inferior quality. Some lines are provided also with rotary side trimmers at the entry end or, more commonly, at the delivery end.
Inspection of the raw pickled product is carried on continuously at the exit end of the pickling lines. Each coil is inspected for surface and edge quality, width and gauge. Some of the defects commonly causing rejection or diversion are as follows: slivers, cracked edges, laminations, off-gauge, off-width, roll marks underpickling, overpickling, handling damage and pitting. However, no systems known to the inventor enable simultaneous inspection of both surfaces the strip subsequent to the pickling operation.
Underpickling results when the steel has not had sufficient time in the pickling tanks to become free of adherent scale and occurs when acid concentration, solution temperatures and line speed are not balanced properly. Variations in the oxide and composition of the steel are also factors in underpickled product, as well as such factors as coiling temperature of the hot-strip mill and inadequate amount of cold working through the processor. Overpickling results from the line delays which permit sections of the steel to remain in the acid too long. The presence of an inhibitor reduces iron loss, but when an inhibitor is not used, iron loss during a short delay period appreciably reduces thickness of the steel and raises the hazard of hydrogen embrittlement. Pitting is related to overpickling, the presence on non-metallic inclusions near the steel surface and to rolled-in scale, slag or a refractory substance. While overpickling is not common in continuous or semi-continuous pickling operations, its occurrence does have a very serious effect on cold-reduction performance and surface appearance of the finished product. Furthermore, product damage from handling or improper equipment adjustment can render the steel unsuitable for further processing.
Prior to recoiling, the pickled steel passes between a set of oiling rolls which cover both surfaces with a small amount of oil. The type of oil used to lubricate the steel, and protect it from rusting during storage and from scratching during handling, is determined by the type of lubricating system on the cold-reduction mill unit. Hence, palm oil diluted with light mineral oil, is applied to the steel at the pickling line when a straight palm oil or a solution contained palm oil is used on the cold-reduction mill. Finally, the pickled and oiled product is recoiled on a conventional up-type or down-type coiler.
Notwithstanding their efficacy in pickling steel strip, such systems are highly complex in construction, undesirably dilatory in oxide removal, and consume copious quantities of materials and energy in operation. Moreover, conventional pickling systems do not permit simultaneous quality inspection of both surfaces of the strip following pickling.
An advantage exists, therefore, for a pickle line for steel strip of relatively compact layout and simplified and efficient operation. An additional advantage exists for a steel strip pickling line which affords simultaneous visual quality inspection of both strip surfaces after the strip has been pickled.